This invention relates generally to equipment for performing stereotactic radiosurgery and more particularly to art apparatus that can be detached and reattached at the same location on a skull to provide a reproducible three-dimensional reference when locating and radiating intracranial and head and neck lesions.
Stereotactic radiosurgery is the practice of gaining precise access to a specific point in the cranium through the application of an external three-dimensional coordinate system. A conventional stereotactic system utilizes a brain mapping technique such as computerized tomographic (CT) scanning or magnetic resonance imaging (MRI), to produce an image representing a "slice" of brain tissue. A series of "slices" constitute a complete study and represent a three-dimensional picture of the brain that defines the relationship of neurological structures and accurately localizes lesions.
The CT or MRI scanning equipment is used in coordination with a frame mounted to a patient's skull by pins or screws. The frame provides a reference that defines the multi-dimensional coordinate system used in identifying intracranial and head and neck lesions. After being attached to the patient's skull, the frame is attached to a platform within the scanning equipment. The frame keeps the skull in the same position during the lesion localization process. The frame remains attached to the patient's skull after localization and through stereotactic radiosurgery to keep the skull in the same relative position in relation to the frame reference points.
There are various surgical and radiosurgical procedures performed on lesions after their localization inside the skull. For example, U.S. Pat. No. 5,027,818 to Bova et. al. describes a radiosurgical method for destroying lesions by directing a radiation beam into the skull. Bova uses a stereotactic frame to assist in localizing the target (e.g., lesion) inside the skull. The intracranial target is then positioned at the focal point of the radiation beam. Multiple radiation pathways are then taken through different areas of the brain all traveling though the same target focal point. Since each radiation pathway is through a different area of the brain, the amount of radiation applied to healthy brain tissue is minimal. At the focal point (i.e., lesion location), however, a very sizable radiation dose is delivered which can, in certain cases, lead to obliteration of the lesion.
The radiosurgical process, in some instances, is a much safer treatment option than conventional surgical methods. It is especially important, however, that the radiation is minimized on certain critical structures inside the skull. For example, when using radiation treatment on a patient's brain, it is important that a minimum radiation dosage be applied to the patient's optic nerves. Therefore, before radiation treatment, the physician must carefully decide on each path the radiation beam will travel through the brain to reach target area. To maintain the same skull reference location, the frame must remain tightly fastened to the skull while the physician is planning this radiosurgical strategy.
To prevent damage to healthy tissue, it would be preferable to apply lower doses of radiation to the lesion over multiple radiation sessions (i.e, fractionated stereotactic radiation therapy). Fractionareal stereotactic radiation therapy, however, is prohibitively expensive and time consuming since the frame must be reattached, and the lesion relocalized before each radiation therapy session. Fractionated stereotactic radiation therapy would be less expensive if the same lesion coordinates could be used for each therapy session. To use the same coordinates for each therapy session, the frame would have to be attached to the skull in the same position in relation to the skull. Present stereotactic immobilization devices or frames, however, cannot be reattached to the skull in the same location with acceptable accuracy. Therefore, the lesion must be relocalized before each radiosurgical session or the radiation beam may have a focal point that is no longer centered on the lesion. An off-target focal point could damage healthy tissue and critical structures in the brain.
The stereotactic frame, while necessary to accurately identify the intracranial target, is time-consuming to attach and is burdensome to carry while attached to a patients skull. For example, correctly fastening the frame to the skull can take one to several hours. Therefore, it is prohibitively expensive to reattach the frame before each radiation therapy session. By necessity, the frame is also large and rather bulky. The large frame is necessary to securely mount the skull to the radiosurgical equipment. Since radiation therapy sessions that are typically performed once a day and continue for several weeks, it is impractical for a patient to carry the frame around on his head throughout the entire radiation therapy process.
To reduce the time and cost of radiation therapy, a single radiation treatment technique or "one-shot" is performed that directs an intense radiation beam at the lesion. This "one-shot" technique in some situations, however, does not destroy a lesion as effectively as fractionated radiation therapy. The high intensity radiation beam also has a greater tendency to damage healthy tissue while traveling through the brain to the lesion. If the lesion is located in certain cranial areas, there is no way to destroy the lesion without also damaging some critical brain structures. Even if a "one-shot" radiation treatment were feasible, the patient must still wear the frame while the physician is localizing the target and deciding upon the various paths the radiation beam will travel to the lesion focal point. During this localization period, the frame applies extreme pressure on the skull. The pressure of the frame is uncomfortable and may cause severe headaches. Thus, regardless of whether fractionated radiation therapy or a "one-shot" radiosurgery process is utilized, it would be beneficial to be able to detach the frame from the patient's skull between the various steps of the stereotactic radiosurgery process.
There are several stereotactic frames used in locating intracranial lesions. However, present frames cannot be attached and reattached to a skull at a reproducible reference location. For example, U.S. Pat. No. 3,357,431 to Newell describes an apparatus fixed to a cranium via screws mounted into the skull. The apparatus in Newell, however, is not used as a three-dimensional reference for CT scanning and only provides a mounting platform for invasive surgical equipment. In addition, the frame of Newell cannot be repeatedly attached to the skull at the same reference position. U.S. Pat. No. 5,176,689 to Hardy et. al. and U.S. Pat. No. 4,923,459 to Nambu, describe apparatus that are attached to the skull to determine the location and size of tumors. However, the apparatus in Hardy and Nambu also can not be detached and accurately reattached to the skull at the same reference location. Therefore, the apparatus described in Newell, Hardy, and Nambu are not useful in solving the problems of time and cost that presently exist with localizing lesions before preforming stereotactic surgical procedures.